1. Field of the Invention
This invention relates to a multi-layered dielectric element for use as a capacitor or an actuator.
2. Description of the Prior Art
In recent years, great attention has been directed to multi-layered dielectric elements and particularly, to multi-layered ceramic capacitors and multi-layered piezoelectric actuators. These elements are fabricated by forming sheets from a slurry of dielectric powder, printing an ink for electrode on the respective sheets, superposing these sheets and sintering the superposed sheets thereby forming a multi-layered element containing electrodes in the inside thereof. The capacitor obtained in this manner has a large capacitance with a small size. When this element is used as an actuator, the generation force increases. The sintering temperature for these elements are generally in the range of from 1250.degree. C. to 1350.degree. C. The sintering at such a high temperature in air disenables inexpensive metals such as Cu or Ni to be used because of the oxidation thereof. Use of expensive metals such as Pd is indispensable, incurring high production costs.
In order to solve the above problem, there has been proposed use, as the internal electrodes, of conductive oxides which are stable at high temperatures. For instance, La.sub.2 NiO.sub.4 having a specific resistance of .about.10.sup.-2 .OMEGA.cm has been proposed for use in capacitors (World Congress On HighTech Ceramics, at Milan on June 24-28, 1986). In this report, BaTiO.sub.3 is used as dielectric layers. La.sub.2 NiO.sub.4 electrodes are sandwiched between the respective dielectric layers, followed by co-sintering at about 1300.degree. C., wherein the interdiffusion of the metal ions at the interface between the dielectric layer and the internal electrode has been studied. When the sintering is contained at 1300.degree. C. for 2 hours, the length of the interdiffusion is about 20 .mu.m. If the BaTiO.sub.3 layer has a thickness of 40 .mu.m or over, this layer is rendered low in resistance owing to the interdiffusion, making it difficult to use the element as a capacitor. For making a thin ion-diffused layer, it is necessary that the sintering be effected at lower temperatures. In addition, it is assumed that since the oxide use has a specific resistance of .about.10.sup.-2 .OMEGA.cm which is higher by three orders of magnitude than metals, high frequency characteristics become poorer. On the other hand, with a piezoelectric actuator, there has been proposed use of a semiconductor of BaTiO.sub.3 to which La is added. In this actuator, BaTiO.sub.3 is used as a piezoelectric or dielectric layer. The semiconductor layers and the BaTiO.sub.3 layers which are alternately superposed are co-sintered at 1270.degree. C. The resultant element has been configured as working as an actuator (Ceramics, Vol. 21, pp. 229, 1986). When the La-added BaTiO.sub.3 semiconductor layers and the insulating BaTiO.sub.3 layers are alternately superposed, a problem involved is the diffusion of La ions alone. In this element, the specific resistance abruptly varies in relation to the concentration of the La ions, thus making a clear interface between the layers. However, the specific resistance of the La-added semiconductor is high at .about.10 .OMEGA..multidot.cm, which places a limitation on the formation of a thin internal electrode layer, i.e. the thickness of 50 .mu.m or below is considered difficult.
For the fabrication of multi-layered dielectric elements, it is essential that a low temperature sintering technique be established in order to reduce the interdiffusion to an extent involving no practical problem and that choice of a material for the internal electrode having a low resistance be made. The sintering of BaTiO.sub.3 used as a dielectric material may be effected at a reduced temperature of from 900.degree. C. to 1100.degree. C. when adding a sintering aid such as LiF or BaLiF.sub.3. Aside from BaTiO.sub.3, it is known that Pb(Fe.sub.1/2 Nb.sub.1/2).sub.0.7 (Fe.sub.2/3 W.sub.1/3).sub.0.3 O.sub.3 having a perovskite structure can be sintered at temperatures indicated above. Accordingly, it is necessary to find out a conductive oxide having a lower specific resistance which is able to be sintered at a similar temperature.